ControlMyMonitor - View and modify the settings of your monitor from command line or GUI
f.lux - f. lux is a piece of software for Windows, Mac, Linux, iOS, and Android that provides a blue light filter over your screen. Research suggests limiting our exposure to blue light in the hours before bedtime can help us fall asleep faster.
PangoBright - Screen dimming software that allows you to control the brightness of your laptop screens as well as external monitors.
Brisync - Synchronize your external monitor brightness with built-in display.
Dimmer - A very small and free utility for Windows to reduce brightness on LCD/TFT screens.
NegativeScreen - Invert your screen colors.